It's very much possible, there is a guide here on how to do it.
http://www.modthegalaxy.com/forums/thread-how-to-add-post-14-1-npc-s-to-the-current-swgemu-core
First post is about the tre's, further down is about the Lua scripts.
Steps needed.
1. Extract new NPC resources from the post Pre-CU tre's.
2. Add CRC's to /misc/object_template_crc_string_table.iff
3. Repack tre file and add to server and client.
4. Create all the Lua scripts for the items you are adding.
5. reboot server.
Should be no need to rebuild when adding new stuff.
Right...... That's the steps in the short version. It's not quite so simple for all items to be added that way. Sometimes CU/NGE asset files need to be edited for them to work correctly with the pre-cu core (Weapons, housing, speeders), but pretty much all things have been cracked now. Check out the guides page for more info, have a go and if you get stuck just ask here or IRC, I'm in IRC 24/7 but not always ATK so just leave a message or PM me